This invention relates generally to ultrasonic welding, and more particularly to a joint design and a method for welding plastic assemblies.
Various methods for welding plastic parts are known. Current ultrasonic welding techniques include spot welding and energy director welding. In a spot welding joint, the parts are stationary during the welding process. As shown in FIG. 1, the weldable component 15 is placed on the base component 10. The ultrasonic welding tool 20 penetrates the weldable component 15 and the base component 10 in order to form the spot weld joint. When filled plastics, such as glass filled nylon and the like, are spot welded, the tool is subject to severe wear. Consequently, the tool has to be made of extremely hard materials, which are often exotic and expensive, and it must be replaced frequently. In addition, with spot welding no space or gap can be tolerated between the parts to be joined. Also, the cosmetic appearance of spot welded joints is undesirable for some applications. Moreover, spot welded joints can lack the strength and mechanical properties required for certain applications.
Other welding processes require relative motion between the parts to be welded. For example, with a joint made using energy director welding, the weldable component moves relative to the base component, as illustrated in FIGS. 2A-B. The weldable component 15 is positioned above the base component 10 with a gap between them. An energy director feature 25 extends downward from the weldable component 15. The ultrasonic welding tool 20 is applied to the upper surface of the weldable component 15 forcing it downward against the base component 10 where the energy director feature forms the weld. Because this method requires that the weldable component move during the welding process, it can result in misaligned welds.
Therefore, there is a need for a welding technique which does not require relative motion during the welding process and which does not penetrate the assembly to be welded.